1. Field of the Invention
The present invention relates to a liquid crystal display control apparatus and a liquid crystal display apparatus and more particularly, to a liquid crystal display control apparatus of a passive matrix type and a liquid crystal display apparatus.
2. Description of the Related Art
In a liquid crystal display apparatus of a so-called passive matrix display type as a super-twisted nematic (STN) type wherein pixels are positioned at intersections between scan and data electrodes perpendicular to each other so that the light transmission factor of the pixel varies with a mean square of a difference between voltages applied to the scan and data electrodes; a drive frame frequency for obtaining the optimum contrast varies with the response time of liquid crystal material.
It is generally believed that the optimum contrast can be obtained when the response time of liquid crystal material (corresponding to an addition of a rise time until display on and a fall time until display off) is 300 ms and a drive frame frequency is between 90 and 120 Hz.
It is also believed that the optimum contrast can be obtained when the response time is 150 ms and the drive frame frequency is 150 Hz or when the response time is 100 ms and the drive frame frequency is 180 Hz or more.
These drive frame frequencies are higher than the drive frame frequencies of 60 to 75 Hz of a cathode-ray tube (CRT) display or thin film transistor (TFT) liquid crystal display.
Accordingly, in order to convert a display signal for the CRT display or TFT liquid crystal display to a display signal for an STN liquid crystal display, it is required to use a frame memory for saving of display data to convert it to a drive frame frequency.
In liquid crystal displays, predominant ones of driving methods for applying binary information (one bit data) of display on and off to the respective pixels of the liquid crystal display.
In order to provide a gray-scale for the liquid crystal display, special processing becomes necessary. As one of systems for implementing this special processing, there is a frame rate control (FRC) system which provides a gray-scale display by setting several frame periods as a unit period and setting the display on/off rate of each pixel in the unit period in terms of unit periods of frame periods.
FIG. 30 is a diagram for explaining an example of gray-scale processing of the FRC system.
In the example shown in FIG. 30, 4 frame periods are set as a unit period, and a pattern of display on and off (referred to as the FRC pattern, hereinafter) is switched on every unit period basis with respect to each certain size of matrix on the display screen.
In a liquid crystal display apparatus of an STN type, a means for implementing the drive frame frequency converting operation and the gray-scale processing operation of the FRC system is generally called liquid crystal controller.
FIGS. 31 and 32 schematically show block diagrams of liquid crystal controllers.
The liquid crystal controller shown in FIG. 31 is of such a type that executes the gray-scale processing operation prior to the drive frame frequency converting operation.
First, for each of colors of red (R), green (G) and blue (B), an input interface 311 accepts gray-scale data (usually, 6-to-8 bit data) of n bits per pixel.
A gray-scale processor 312 then executes the gray-scale processing operation of the FRC system according to the gray-scale data received from the input interface 311 to generate of one bit of indicate on/off data, and writes it into a frame memory 313.
Thereafter, the indicate on/off data are read out from the frame memory 313 in synchronism with the drive frame frequency of the liquid crystal output display data to be converted to a frame frequency, and then output to an STN liquid crystal display (not shown) through a liquid crystal output interface 314.
The liquid crystal controller shown in FIG. 32, on the other hand, is such a type that executes the frame frequency converting operation prior to the gray-scale processing operation.
First, for each of the colors R, G and B, an input interface 311 accepts gray-scale data (usually, 6-to-8 bit data) of n bits per pixel. After that, the gray-scale data are written into a frame memory 313.
Next, the gray-scale data are read out from the frame memory 313 in synchronism with the drive frame frequency of the liquid crystal output display data to be converted to a frame frequency, and thereafter a gray-scale processor 312 executes the gray-scale processing operation of the read gray-scale data to generate one bit of indicate on/off data.
And the gray-scale processor 312 outputs the indicate on/off data to an STN liquid crystal display (not shown) through a liquid crystal output interface 314.
Disclosed in Japanese Laid-Open Publication No. 8-87247 is a technique for displaying a video signal not conforming to a liquid crystal display of the passive matrix type.